Nothing would happen. However, what could happen is it'll make it easier for someone to hack your computer. But that's a worse case scenario and you can easily close the ports you opened.
Closing your ports is fairly easy. Go back into your router, go to "Setup/Configuration" > "Advanced Setup" > "Begin Advanced Setup" > "Application Level Gateway" > Select the "User" radio button under the "Category" column > Look in the "Applied Rules" box and select the row that you created following the instructions in the link I provided > While it's selected, click the "Remove" button > And finally, click "Save and Restart".
Now, rather than removing the ports you set up, I instructed you to "deactivate" them. This way, you don't have to go through the instructions each and every time you need to forward your ports:
When you want to forward your ports again, simply go back through until you get to the "Application Level Gateway" screen. Make sure the "User" radio button is selected in the "Category" colum and select the row you created and click the "Add" button. Then finish by clicking the "Save and Restart" link.